The present invention relates, in general, to micromirrors and to arrays of such micromirrors, and more particularly to a low voltage, single crystal silicon micromirror assembly having a high fill factor, and to methods of fabricating such micromirrors and arrays.
Significant applications of microelectromechanical structures (MEMS) arise from the fact that the fabrication process for such devices allows individual microactuators to be organized into massive arrays which cooperatively perform a macroscopic function. A prominent example of the application of MEMS to such arrays, in which interconnected microactuators are used, are digital projection displays based on digital micromirror devices (DMD). This technology has been used in several applications, including fiber optic crossbar switches, wave front correctors used in adaptive optics and free space communication, optical beam steering, and variable optical gratings. The DMD technology is based on a released polysilicon thin film and metal micromachining process, and on flexible dielectric membrane fabrication using KOH wet silicon chemistry.
Mirror surfaces can be produced on such flexible dielectric membranes, and these have several advantages over conventional piezoelectrically-actuated mirrors. Fabrication is made considerably easier, because no discrete assembly is required, and the actuators for the mirrors can be integrated on the same chip. This process utilizes existing semiconductor fabrication technology to take advantage of batch fabrication processes so that the final cost is low. Performance is enhanced because the system is operated at low voltage and low power. Because the process permits a high actuator density, high spatial resolution is available, and the arrays are lightweight so that high frequency operation is possible.
The biggest drawback of the deformable mirrors produced by this thin film technology is a residual stress in the deposited thin films as well as a stress interaction between the substrate films and reflective mirror films. A polysilicon mirror deformation of 50 nm after the release of the structure has been reported. This produces deviations from the flatness required for a mirror surface, and although it is relatively small compared to the thickness of the mirror and the overall motion of the mirror that can be produced by actuators, such deviations represent significant fractions of the wave length of visible light and, therefore, adversely affect the performance of the mirror. Because of this residual stress problem, the rectangular size of such a mirror is limited to about 200 micrometers or less on each side. To relieve some of this stress, the mirror surface is often fabricated with apertures, which also help in the release process, but the resulting surface of the thin film can be optically rough, although post-deposition treatments can minimize this surface roughness.
Another difficulty with such thin film devices is that they are normally driven by capacitive forces, with the result that motion in the vertical, or z-direction, is based on parallel plate forces. As a result, the deflection of the mirror is not controllable if the motion exceeds about one third of the initial parallel plate gap.
Prior mirror actuators normally include a spring system for motion in response to signals supplied to the capacitor plates, and the design of such spring systems plays an important role in lowering the operating voltages and minimizing actuator area. A variety of spring designs have been developed, some using flexible torsional hinges as the spring system, with the hinges being hidden underneath the mirror structure. Deformable micromirror arrays composed of a flexible polysilicon membrane supported by an underlying array of electrostatic parallel plate actuators can provide extremely high fill factors, in the range of 90%. However, where the thin membrane acts as the spring system for the mirror structure, up to 150 V may be required to achieve a center deflection of 1 xcexcm, due to the typical geometry of the membrane. Further, the deflection of the membrane may not be uniform, resulting in nonuniform vertical motion across the mirror surface. Tests of arrays using folded flexures attached to the moving mirrors produced an overall effective spring constant in the z-direction that was less than that of the membrane system so that a considerably lower operating voltage was achieved. However, both the flexures and the mirror had to be fabricated in the same layer so that any area taken up by the flexures would directly reduce the area of the array covered by the movable mirror surfaces. Thus, a low fill factor of only about 40% was achieved with this design.
The foregoing difficulties are overcome, in accordance with the present invention, by a micron-scale, single crystal silicon (SCS) micromirror assembly including a mirror platform having an optical surface, which is optically flat and smooth, free of residual stress, and which is highly reflective after the deposition of a thin metal layer. The assembly also includes a high aspect ratio MEMS actuator structure which supports the mirror platform and produces enhanced manipulation of the optical surfaces.
In accordance with a preferred form of the invention, a suspended, or released, drive actuator is fabricated in one surface of a double polished wafer, with the drive actuator supporting, and being precisely aligned with a corresponding micromirror platform structure fabricated in the opposite surface of the wafer. The polished wafer surface in which the mirror platform is fabricated provides an optically flat mirror surface for receiving a reflective coating such as a thin metal layer, or multiple layer thin films. The mirror assembly is fabricated in the wafer by a suitable process, such as the Single Crystal Reactive Etch And Metallization process (SCREAM) process described and illustrated in U.S. Pat. No. 5,426,070 to Shaw et al, issued Jun. 20, 1995, and is released from the wafer by a through wafer etch. The actuator is connected to the back of the corresponding mirror platform by rigid mounting posts to transfer the motion of the actuator to the mirror.
Micromirror assemblies may be fabricated in arrays of any desired size, utilizing known fabrication techniques, with the individual actuators being operable through individually addressable electrical connections. Routine silicon patterning can be done on the optically flat, SCS mirror surfaces for various optical applications, and scaling up of the arrays may be done.
The actuator which supports the mirror platform preferably utilizes an asymmetric comb finger design having interdigitated stationary and movable fingers having high aspect ratios. Vertical motion of the micromirror assembly with respect to the surface of the substrate is produced by applying a net electric field between adjacent fingers. By providing comb finger electrodes having different heights the range of motion limitations of other parallel plate actuator configurations are avoided.
The MEMS single crystal silicon micromirror platform provided by the present invention has sufficient thickness and rigidity to permit fabrication of features such as optical gratings on the platform and the mirror assembly has an excellent structural rigidity, to provide a uniform motion across the optical surface of each mirror upon operation of its corresponding actuator. By providing an array of mirror platforms on one side of a wafer and providing the corresponding actuators, contact pads, address metal lines and related structural features on the other side of the wafer, a high fill factor can be attained for the mirror array; that is, up to about 90% of the array surface is mirrored, with the remaining portion being taken up by the spaces between adjacent mirrors.
A MEMS micromirror assembly in accordance with the present invention is fabricated in a single crystal silicon (SCS) substrate or wafer using, in one embodiment, a two-mask process. The SCS wafer is polished on its upper and lower surfaces and both surfaces are covered by an oxide layer. If the mirror is to be a reflective metal, for example, the bottom surface is coated with a metal layer such as aluminum, and thereafter a first mask defining a micromirror platform is lithographically defined on the bottom surface. A trench surrounding the platform is etched through the mask, through the aluminum, and partially through the wafer. The mirror surface remains covered by the mask during the succeeding steps, so it is not damaged during formation of the actuator structure and release of the mirror platform.
Thereafter, an actuator, or micromirror drive structure, which may be in the form of a comb-type capacitive drive, is fabricated in the top surface of the wafer. In this embodiment, the top surface oxide layer is replaced by a second oxide layer in which is photolithically defined the actuator pattern in careful alignment with the previously formed micromirror platform using a second mask. The actuator pattern is transferred to the silicon wafer by etching, and in accordance with the SCREAM process, the walls of the resulting trenches are covered with a conformal oxide layer. The oxide is removed from the floor of the trenches and an isotropic etch is used to release narrow actuator beam structures. Another conformal oxide layer is applied, the floor oxide is again removed, and the trenches are deepened. Thereafter, another isotropic etch is used to release wider actuator beam structures, and the actuator is metallized to form adjacent capacitive drive electrodes. By selecting the relative widths of the beam structures, released beams of varying heights and aspect ratios can be produced, so that an asymmetric comb-type drive is formed by interdigitated movable and stationary fingers of different heights.
Following formation of the actuator drive structure, the trench in the bottom surface surrounding the mirror platform is etched through the wafer to release the micromirror.
In a second, three-mask embodiment, the actuator is fabricated using second and third masks, following fabrication of the micromirror platform on the bottom of the wafer using a first mask, as described above. In this embodiment, the oxide layer on the top surface of the wafer is patterned through the second mask to define an area where selected portions of the actuator are to be fabricated; for example, where the fixed fingers of a comb-type actuator are to be located. This pattern is transferred to the top surface oxide layer, which is selectively etched to reduce the thickness of the oxide layer. The third mask is then used to define the pattern of the entire actuator structure, and this pattern is transferred into the silicon by the SCREAM process. The structure is then released and metallized, to produce released metallized beams with selected heights, as determined by the thickness of the top surface oxide layer on each beam. Thereafter, the bottom trenches are etched through the wafer to release the micromirror structure, as previously described.
Multiple adjacent micromirrors with corresponding actuators may be fabricated in a single wafer, using the foregoing process, to form an array of MEMS micrometer-scale micromirrors which cooperate to produce a macro-scale mirrored surface. Each micromirror assembly in the array is individually movable and controllable.
The micromirror platform fabricated in the bottom surface of the wafer may be of any desired shape, and thus may be generally rectangular, is relatively thick so as to be sufficiently rigid to maintain optical flatness when the platform is moved by its actuator and to permit fabrication of optical gratings on it, and has dimensions in the micrometer scale. The platform is surrounded by a narrow through-wafer trench and by sufficient space to permit routing of metal connector lines, and these separate the micromirror from a surrounding substrate or, in the case of an array, from adjacent micromirrors. Each micromirror platform is attached to, and supported by, a corresponding controllable actuator structure which is fabricated in the top surface of the wafer and supported by torsion bars or springs.
The actuator, in one form of the invention, includes a backbone structure which may incorporate plural longitudinal, parallel, high aspect ratio beams extending parallel to the length of the micromirror, with multiple transverse beams interconnecting the longitudinal beams in a ladder-like structure.
In one embodiment of the invention, the backbone is connected for pivotal motion about a single high aspect ratio torsion bar which is perpendicular to the backbone, with opposite ends of the torsion bar being anchored to the substrate. In a second embodiment, the backbone is connected to two spaced high aspect ratio torsion bars, with a pair of spaced hinges, each including two stress-relieving bars, being located in the backbone between the torsion bars to divide the backbone into three segments. The outer ends of both torsion bars are anchored to the surrounding substrate to support the backbone in a cavity in the substrate. The two hinges are transverse to, and are coplanar, with the backbone and are generally parallel with the torsion bars. These hinges permit the center segment of the backbone to move uniformly in a z-direction, perpendicular to the plane of the torsion bars and to the plane of the wafer surface, while allowing the two end segments to pivot around their respective torsion bars to permit the out-of-plane motion of the center segment.
Connected to the backbone, and to the middle segment of the backbone in the second embodiment, is a comb-type actuator consisting of multiple movable, high aspect ratio actuator fingers extending perpendicularly from the backbone, and multiple stationary, high aspect ratio fingers mounted on the surrounding substrate and interdigitated with the movable fingers. The individual fingers have high height to width ratio to provide relative stiffness in the vertical direction, and this may have widths on the order of 0.5-3 xcexcm and heights on the order of 5-100 xcexcm or more. Integral with the backbone are vertical support posts which extend downwardly through the wafer, the lower ends of the posts being connected to, and integral with, the mirror platform formed in the bottom surface of the wafer.
The movable actuator fingers connected to the backbone are fabricated as described above to have a different height than that of the interdigitated fixed fingers. Both sets of fingers are metal-coated on their tops and sidewalls to provide actuator electrodes, and the difference in the electrode heights produces a vertical asymmetry in the electric field between the stationary and movable fingers, when a potential difference is applied across the fingers, as described in U.S. Pat. No. 6,000,280, issued Dec. 14, 1999. The asymmetric electric field is mainly due to the difference in the height of the metal covering of the adjacent fingers, in known manner. As described in the aforesaid patent, the asymmetric electric field distribution results in an out-of-plane actuation force that causes the movable fingers, and thus the backbone, to move in a vertical direction upon the application of voltages to the electrodes. In the first embodiment of the invention, where the backbone is connected to a single torsional bar, a voltage across adjacent electrodes produces relative vertical motion of the movable fingers with respect to the stationary fingers, and this causes the cantilevered backbone to pivot about the axis of the torsional bar. This torsional operation is converted into a pure z-direction motion in the second embodiment by connecting both ends of the backbone to torsional bars and incorporating the stress-relieving hinges discussed above, thereby allowing accurate vertical translation of the mirror out of the plane of the bottom surface of the substrate.
In accordance with the invention, the described mirror structures may be fabricated in arrays on a wafer, with any practical number of mirrors being provided.